Demand for communications between the road administration and ground vehicles tends to grow even more in the future. Particularly, drivers on highways need frequent information exchanges between the stationary station and the mobile stations in order to drive under less load and to prevent accidents. A developed form of such a system is exemplified by an automatic driving system with extensive provision of various sensors and cameras on both the road and vehicle for close communications between the stationary station and vehicle (see, for example, Japanese Unexamined Patent Publication No. 8-241495 (1996).
The consideration of future expansion of the automatic driving system dictates the need to construct a driver support system (hereinafter referred to as “roadway communication system”) based on the communications with vehicles. Preparatory to the construction of such a system, communication areas (cells) must be provided over the roads.
It may be contemplated to lay leakage coaxial cables along the roads, which involves a large scale cable-laying work. Besides, the leakage coaxial cables need be embedded at a relatively great depth in the roads and hence, the wave energy presents a disadvantageously small range with respect to a transverse direction of the lane.
In contrast, as shown in FIG. 28, a system with road antennas 120 disposed at given space intervals along the road for communications permits one road antenna 120 to define a relatively broad cell 121. In this case, the road antennas are each connected to a respective control station of the road administration via an optical fiber, coaxial cable or the like.
(A) In a case where the road antennas are disposed, a large vehicle approaching a small vehicle may sometimes cut in a line of sight of the small vehicle. FIG. 29 illustrates a particular state where the small vehicle is in a wave blocking area of the large vehicle. The microwaves and millimeter waves at high frequencies have small diffraction angles and hence, are prone to be blocked. This results in a breakdown of roadway communications.
It is therefore, an object of the invention to provide a roadway communication system accomplishing seamless communications between the stationary station and the mobile station.
(B) The installation of the road antennas generally entails inter-carrier interference or inter-symbol interference associated with the occurrence of delayed multipath waves, causative factors of which are structures near the road or plural vehicles in the cell which reflect the waves. When the inter-symbol interference occurs, a bit error rate is not improved even if the waves are received at high reception levels. This leads to a so-called floor error.
In the mobile communication system based on a single carrier, a receiver is generally equipped with an equalizer having inverse characteristics of those of a transmission line thereby to eliminate the effect of the inter-symbol interference associated with the delayed multipath waves.
Unfortunately, the automobile travels through the cell at such high speeds that the radio frequency energy field presents too sharp fluctuations per unit time for the equalizer to cope with the calculations. Thus, it is impossible to transmit signals at less than a given transmission error rate. Additionally, a large-scale hardware is required for implementing the equalizer, which results in great power consumption.
On this account, the invention has an object to provide a roadway communication system capable of preventing the inter-carrier interference and inter-symbol interference for accomplishing stable communications between the stationary station and the mobile station.